Sponsor definition: N a person or organization that pays for or contributes to the costs involved in staging a sporting or artistic event in return for advertising:

V provide funds for (a project or activity or the person carrying it out):

SOURCE

Donation definition: something that is given to a charity, especially a sum of money

Sponsoring something means you are pledging the money. It is a contractual obligation IMO if you sponsor something with a prize or with money. If he had used the word donation then he has every right to change the amount. I think he is breaching a contract and deserves feedback honestly. Had he decided after a day or week to change the amount or not to sponsor, then he would prob be fine, but to wait til nearly over and stop the sponsorship because of his drama on the forum is ridiculous.